A vehicle where a battery is charged by regenerative braking, stores kinetic energy of a driving vehicle in a lead-acid battery. In the vehicle performing regenerative power generation, the lead-acid battery is charged through driving an alternator by kinetic energy of the vehicle at deceleration. As an internal resistance of the lead-acid battery at charging, namely a charging resistance is large, there is a demerit that the lead-acid battery cannot be efficiently charged with regenerative generation power. As a charging current of the lead-acid battery at regenerative braking is remarkably large, for example, 100 A or more, the lead-acid battery is frequently charged with a large current by regenerative generation each time the vehicle is braked, and then there is a demerit that a life of the lead-acid battery is remarkably shortened.
In order to resolve this demerit, the power supply device where the lead-acid battery is connected in parallel to the sub-battery, is developed. (refer to patent literatures 1 and 2)